For the purpose of owning, operating, constructing, and equipping a sewage disposal plant or sewerage system or improving or extending such existing system as provided in section 18-501, a city or village may issue revenue bonds therefor. Such revenue bonds, as provided in this section, shall not impose any general liability upon the city or village but shall be secured only by the revenue of such utility as provided in section 18-504. Such revenue bonds shall be sold for not less than par and bear interest at a rate set by the governing body. The amount of such revenue bonds, either issued or outstanding, shall not be included in computing the maximum amount of bonds which such city or village may be authorized to issue under its charter or any statute of this state.